Tripod type constant velocity joints are well known in the automobile industry, for example, as one type of constant velocity joints used in the drive system of automobiles to transfer a uniform torque and a constant speed, while operating with a wide range of joint angle.
As illustrated in FIGS. 1(a) and 1(b), a tripod constant velocity joint consists generally of a tripod housing 1′ having a first rotating shaft 1″ extending in a longitudinal axis direction (i.e., X-X axis direction) for transferring the torque of an engine through a coupling means (e.g., an external serration 1a′) to transmit the engine torque T1 to a second rotating shaft 5′ to drive the same. The tripod housing 1′ includes three grooves 1b′ to retain therein and guide a torque transmitting members of the second shaft 5′ (e.g., three spherical rollers 2′, a plurality of needle rollers 3′) in order to accommodate smooth rotating of respective spherical roller 2′ relative to a direction generally perpendicular to the longitudinal direction, namely, Y-Y axis direction shown in the drawings. Each spherical roller 2′ is rotatably coupled to a spider 4′ to transmit the torque to the shaft 5′, and the spider 4′ includes a boss portion 4a′ with an internal serration 4b′ to transmit the torque there-through, and three trunnions 4c′ each extending radially from the boss portion 4a′. Each trunnions 4c′ is coupled with a retaining outer 6′ to retain the needle rollers 3′ in the outer circumference of its corresponding trunnions 4c′ of the spider 4′, and a clip 7′ mounted in the hole formed in the trunnions 4c′ to support the retaining outer 6′.
In the tripod constant velocity joint shown in FIGS. 1(a) and 1(b), if the driving torque T1 from the engine of vehicle is transmitted via the external serration 1a′ of the shaft 1″, the torque T1 is transferred to the spherical roller 2′ through the housing groove 1b′. As a result, a force F1 is applied between needle rollers 3′ and trunnion 4c′, in which the force F1 can be calculated by the equation of T/3PCR, where the PCR is a pitch circle radius of tripod housing groove 1b′ that is measured from the center of the spider 4′ or the first shaft 1″ to the center of the housing groove 1b′ as shown in the drawings. This force F1 produces a reaction torque T5 on the driven shaft 5′ through the internal serration 4b′ of spider 4′ and the external serration 5a′ of shaft 5′.
FIGS. 2(a) and 2(b) illustrate weak portions in the tripod joint which should be considered in the joint design in order to provide a tripod constant velocity joint having a desirable strength in terms of a torsional strength and a durability (or torsional fatigue) in particular. In the tripod joint design, the shortest or minimum diameter SD of shaft 5′ is typically considered to be the weakest portion in design in terms of torsional strength, and the torsional strength is generally defined as the measured torsional strength when the joint breaks upon subjecting to a predetermined amount of force or torque. However, the compression stresses on the contact surface W1 between the spherical roller 2′ and the guide groove 1b′ of the housing, the contact surface W2 between the needle rollers 3′ and the spherical roller 2′, and the contact surface W3 between the spider trunnion 4c′ and the needle rollers 3′ can also be considered as weak portions affecting the joint life, namely, the durability or torsional fatigue which can be typically expressed as the torque to bring the flaking damage on the contact surfaces within the required or desired life of the vehicles. In addition, the bending stress on the root section W4 of spider trunnion 4c′, the tensional stress on the section W5 from the concaved corner portion 4r′ (between the root of trunnion 4c′ and the adjoining boss portion 4a′ of spider 4′) to the major diameter SMD measured between two diametrically-opposite valley portions of internal serration 4b′ of spider 4′, and/or a hoop stress on the section W6 from the boss diameter SBD to the major diameter SMD of spider 4′ are also known as main failure portions against the torsional fatigue upon subjecting to the oscillating sinusoidal torque which leads the joint to break.
As described, the strength of tripod joints is typically determined by the minimum diameter SD of shaft 5′ because this portion is designed to break first for most torsional strength tests. Therefore, in the compact tripod joint design, the dimensions related to the above weak points should be considered to have the strength at least the same as or greater than the strength of the shaft 5′ for the joint life test and torsional fatigue test.
With regard to the life of the joint which can be explained as a rotational durability of internal parts of the tripod joint for the required vehicle life, contact stresses between the spider trunnion, needle rollers, spherical roller, and groove surface of tripod housing should be considered. When the pitch circle radius PCR of the tripod joint is reduced to provide a more compact joint, the stresses between the internal parts of the joint become increased by the increase of the force F1 on the contact surfaces as the result of the reduction of the pitch circle radius PCR. Therefore, considering the stress increase between the internal parts, the limitation in the reduction of pitch circle radius PCR is another important factor for the design of compact tripod joint.
Regarding the torsional fatigue which is the joint durability against the repeatedly applying torque, the strength of spider should also be considered as an important factor. In order to obtain a compact design of the tripod joint by reducing the external diameter of tripod housing, the pitch circle radius PCR of the tripod housing should be reduced, and this results in the increase of the reaction force F1. However, as the increased force F1 cause to increase the bending stress on the root area W4 of the spider trunnion and tensional stress on the corner radius portion 4r′ of the spider, the reduction of pitch circle radius RCR should be limited to a certain degree due to the stress requirements in the torsional fatigue tests of the tripod joint.
Moreover, in order to maximize the effect of the compact tripod joint design, due considerations should also be given to the reduction of key dimensions in other parts as well, such as the length of needle rollers and the width of the spherical roller, in addition to the reduction of the pitch circle radius of the joint. While reductions in the length of needle rollers and the width of spherical roller are necessary to reduce the external diameter of tripod housing, such reductions lead to the degradation of the stress requirements between the needle rollers and trunnion spider, the needle rollers and spherical roller, and also between the spherical roller and groove surfaces of the tripod housing. Therefore, an optimization in the relevant dimensions is very important to provide a compact tripod joint which is also durable with required strength.